Control devices for cold-working structures

ABSTRACT

Control systems for indenters used in cold working processes, and in particular, for StressWave cold working of metal workpieces. Load on an indenter is increased by increasing the force applied, such as by pressure in a hydraulic reservoir acting on a piston. When the load reaches a desired limit, as may be calibrated by the pressure in the fluid reservoir responsive to the piston being driven by the indenter, then a pressure relief valve allows the fluid to flow from the cylinder to a hydraulic reservoir, thus allowing the indenter shaft to move away from the workpiece. Then, on recycle of the apparatus, an automatic control system reacts to such movement and pumps fluid back to the reservoir, and thus returns the indenter shaft to a pre-determined position. In one embodiment, the pressure relief valve is integrated into, or attached to, the indenter. Other embodiments are disclosed for control of various compound or simple indenters.

RELATED PATENT APPLICATIONS

[0001] This patent application claims priority from prior U.S. Provisional Patent Application Serial No. 60/361,977 filed on Mar. 5, 2002, entitled Control Devices for Cold Working Structures, the disclosure of which is incorporated herein in its entirety by this reference.

TECHNICAL FIELD

[0002] This invention relates to the control of indenters used for placing dimples in metal workpieces in a process for imparting fatigue life improvement at bounding regions at or near holes or other features in such metal workpieces. More particularly, the invention relates to specific methods for control of variations on a process of indenting such metal workpieces, to maintain uniform quality and to assure that the desired residual compressive stress is uniformly achieved in the metal object workpieces.

BACKGROUND

[0003] Metal fatigue is the damage or failure of a metal structure that is caused by cyclic tensile loads acting on the structure. Fatigue cracks generally start at the location of highest stress within the structure. Typically, this occurs at a change in section of the structure. A typical change in section would be a fillet radius, bend, hole, notch, or other cutout. The change in section causes the otherwise uniform tensile stresses to be concentrated at the location, thus providing a potential weak spot in the structure. Geometric features that increase the local stress in this manner are called stress risers or stress concentrators. By far the most common stress concentrator is a hole. Open holes concentrate the uniform stress by a factor of three or more at their periphery. Improving the fatigue life of holes has been the subject of much research.

[0004] One of the more successful methods for improving the fatigue life of holes in metal structures is cold-working. The term cold-working covers a number of processes that impart beneficial compressive residual stresses around the hole periphery. Hole cold-working methods improve fatigue life by producing residual compressive stresses around the hole. These compressive stresses reduce the magnitude of the tensile stress at the hole periphery thereby increasing its fatigue life.

[0005] The StressWave cold-working process is unique among hole cold-working processes in that it imparts residual stresses into the metal, using devices called indenters, before the hole is drilled. The advantages of this process are described in U.S. Pat. No. 6,230,537, the disclosure of which is incorporated herein by this reference. A uniform pressure profile indenter and its equivalents are found in U.S. Pat. No 6,230,537.

[0006] Another method for cold-working structure using the StressWave process is found in co-pending U.S. patent application Ser. No. 09/782,880, filed Feb. 9, 2001, the disclosure of which is incorporated herein by this reference. This latter patent application discloses the use of a compound indenter having a primary indenter portion, a secondary indenter portion, and from zero up to N additional indenter portions. Additionally, a foot portion can be provided adjacent to one or more of the indenter portions.

[0007] Cold-working of metal structure via the StressWave process provides a method for significantly increasing the fatigue lives of holes. Assembled structures with a large number of fatigue prone holes can benefit by automating the cold-working process to decrease cost, improve quality, and increase structural integrity. However, in order to implement the StressWave cold-working of structures for improved fatigue life in industrial fabrication facilities requires a means for controlling the indenters used to indent the workpieces. It would be desirable to provide a system for controlling the indenting process to ensure that it is performed to specification and completed in a consistent and timely manner.

BRIEF DESCRIPTION OF THE DRAWING

[0008] In order to enable the reader to attain a more complete appreciation of the invention, and of the novel features and the advantages thereof, attention is directed to the following detailed description when considered in connection with the accompanying drawings, wherein:

[0009]FIG. 1 shows the free body diagram and load-displacement curve of an integral one-piece compound indenter or solidly attached compound indenter components. R1 and R2 are the reaction forces produced by the primary and secondary indenter respectively as a result of the applied load.

[0010]FIG. 2 shows the free body diagram and load-displacement curve of an adjustable compound indenter where a spring is attached to the secondary indenter. R1 and R2 are the reaction forces produced by the primary and secondary indenters respectively as a result of the applied load. K2 is the spring constant for the spring attached to the secondary indenter.

[0011]FIG. 3 shows the free body diagram and load-displacement curve of an adjustable compound indenter where a spring is attached to the primary indenter. R1 and R2 are the reaction forces produced by the primary and secondary indenter respectively as a result of the applied load. K1 is the spring constant for the spring attached to the primary indenter.

[0012]FIG. 4 shows the free body diagram of an adjustable compound indenter where a loading or positioning device is attached to the secondary indenter. R1 and R2 are the reaction forces produced by the primary and secondary indenter respectively as a result of the device and/or applied load.

[0013]FIG. 5 shows the free body diagram of an adjustable compound indenter where a loading or positioning device is attached to the primary indenter. R1 and R2 are the reaction forces produced by the primary and secondary indenter respectively as a result of the device and/or the applied load.

[0014]FIG. 6 shows the free body diagram of an adjustable compound indenter where loading and/or positioning devices are attached to both the primary and secondary indenters. R1 and R2 are the reaction forces produced by the primary and secondary indenter respectively as a result of the devices and/or applied load.

[0015]FIG. 7 shows a schematic of the various components of an indenter control system including indenters, driving head, driver controller, frame, indenter controller, indenter controller software, sensors, measuring devices, and statistical process control software.

[0016]FIG. 8 shows a dependently controlled compound indenter having a spring connecting the primary indenter to the secondary indenter

[0017]FIG. 9 shows a dependently controlled compound indenter having a fluid transferring load or pressure from the primary indenter to the secondary indenter.

[0018]FIG. 10 shows a dependently controlled compound indenter having a spring connecting the primary indenter to the secondary indenter where the spring substantially fills the cavity.

[0019]FIG. 11 depicts a dependently controlled compound indenter having an internal safety stop and having a spring connecting the primary indenter to the secondary indenter

[0020]FIG. 12 shows a reduced profile dependently controlled compound indenter having an internal safety stop and having a spring connecting the primary indenter to the secondary indenter

[0021]FIG. 13 shows the control limits as represented by the dotted lines, for load, displacement, or combined load and. displacement for a typical load-displacement curve for a compound indenter.

[0022]FIG. 14 shows the hydraulic control schematic for a system of controlling indenters in the cold working of metal workpieces, including a partial sectional view of a compound indenter that can be used to practice the methods taught herein.

[0023] The foregoing figures, being merely exemplary, contain various elements that may be present or omitted from actual implementations of the method(s) disclosed, depending upon the circumstances. An attempt has been made to draw the figures in a way that illustrates at least those elements that are significant for an understanding of the various embodiments and aspects of the invention. However, various other elements of the exemplary methods provided, especially as applied for different applications and variations of the fatigue life enhancement processes described, may be utilized to provide an advantageous manufacturing method for providing fatigue life enhanced products such as aerostructures.

DETAILED DESCRIPTION

[0024] Various Indenter Designs and Generalized Control Techniques

[0025] The components of a compound indenter may be in an integral one-piece combination. Or, one or more components may be adjustable in a compound indenter. The various indenter portions may be controlled separately, independently or dependently. The cross sectional shape, working length and other features of the primary indenter portion are dependent on the material properties of the workpiece structure, the hole diameter and shape, and the number and thickness of the layers comprising the structure. A compound indenter that allows for variation and control of one or more of its components is considered an adjustable compound indenter. Generally, the working length of the primary indenter is the component that is adjusted.

[0026] An indenter control system for controlling the action of indenters may include the indenter's driving head, driver controller, frame, indenter controller, indenter controller software, sensors, measuring devices, and statistical process control software. A system for controlling the indenters includes all or a selected number of these components. The indenter control system ideally controls the process parameters including load and displacement. A system with a high level of control and monitoring with feedback controls the magnitude, direction, rate, and change of rate of any of these process parameters. For purposes of definition the term load applies to any force, stress, pressure, weight, or energy used to actuate any and all components of an indenter. Further, the term displacement applies to any strain, motion, or position used to actuate any and all components of an indenter

[0027] Indenters are generally attached to the driving heads of automated fastening and assembly equipment such as provided by Brotje, Gemcor, Electroimpact, and the like. The driving heads are further attached to the frames of said units. The frame carries the reaction forces of the indenter load or displacement during indenter actuation. For the case of the Electroimpact equipment the frame secures and positions the driving head while the inertia force of the indenter actuation is carried and dampened within the driving head.

[0028] Indenters can also be attached to the driving heads of hydraulic presses, pneumatic presses, roller screw devices, screw presses, etc. In each instance the driving head to which the indenter is attached provides the load and/or displacement while the frame carries the reaction load during the actuation of the indenters.

[0029] The StressWave cold-working process may also be used on hand-held devices. The tools and devices may be hydraulic, pneumatic, electric or any other tool that provides a load or displacement to at least one of the indenters. In the case of heavy hand-manipulated devices a frame may be defined as a cabling and counterweight system or any other device that allows for ease of manual movement and positioning. These hand-held tools rely on, for example, electromagnetic release of energy to provide a rapid means of cold-working the structure. For short duration pulse loads the inertial mass of these hand-held tools may react and damp out the reaction force of the cold-working so that the tools do not generally require a frame. Another type of hand-held tool is one where force is applied to one side of the structure and reacted through a small c-yoke tool. These tools have in some instances a limitation of working only those holes near a free edge of the workpiece.

[0030] Each of the aforementioned equipment, devices and tools, here combined as an indenter, driving head, frame, driving head and indenter control software, and sensors has various levels of sophistication when it comes to control. Together as an indenter control system each of the applicable devices has a means of applying loads or displacements either independently or in combination. This high degree of control allows for StressWave cold-working a multitude of single and multiple element structure with varying stack thickness, different materials, and stack balance. Further, StressWave cold-working on each side of the structure can be done at unequal levels of treatment, unequal dimple depths, and with different indenters.

[0031] Some devices or equipment have a means of monitoring and controlling not only the magnitude of the load and displacement, but also the rate and the change of rate with which they are applied. Still other equipment has the capability of controlling and monitoring the full x, y, and z position of the driving head and consequently the indenter relative to the workpiece. Equipment outfitted with the proper monitoring and measuring devices have a means for measuring the effect of the load and/or displacement on the workpiece such as in the case of measuring the head height of a driven or squeezed rivet or the depth of a dimple produced by the StressWave process.

[0032] The most capable and sophisticated indenter drivers have the ability to provide fully independent control of indenters and the various components of adjustable compound indenters. Such driving heads and their controlling software and sensors allow for independent application, measurement, and control of load and displacement for the primary indenter, the secondary indenter, up to N additional indenters, and the foot if present. This type of control is called “independent control”. This control uses as a reference various parameters such as time, load, and displacement. In this way control for any application can be achieved using any variation and combination of load, displacement, and time such as load versus time, displacement, displacement versus time, and load versus displacement. The loads and/or displacements acting on the indenter or components of an adjustable compound indenter may be applied with a constant rate of increase or decrease or with any other profile desired that maximizes fatigue life improvement and/or improves production efficiency. It may be desired to have different load or displacement profiles for actuating the indenters into the structure and removing the indenters from the structure. Equipment and devices capable of fully independent control may be fitted with an adjustable compound indenter that provides the overall system maximum flexibility for cold-working structure.

[0033] The driving heads to which the indenters are attached may actuated on one or both sides. In a squeeze type device there is typically a driving head on only one side. The side opposite the driving head holds the other indenter in a fixed relationship to the frame and reacts the driving head force. When cold working the driving head moves the indenter toward the workpiece until it makes contact. If the fixed indenter is already in contact with the workpiece then the cold working procedure may commence. If the fixed indenter is not in contact with the workpiece then the driving head moves the indenter and workpiece until the workpiece makes contact with the fixed indenter. When both indenters are touching the workpiece the cold working procedure can commence.

[0034] Sensors and measuring devices that monitor the loads and displacements provide important feedback to the indenter control system. The information acquired by the sensors may be the magnitude, direction, rate, and change of rate of the load and/or displacement. The indenter controller software, database, or controlling hardware may take corrective or adjusting actions based on the values or deviations from a prescribed value or set of values. Baseline values are obtained by applying an increasing load or displacement to a specific indenter configuration, material, thickness, and structural geometry. In this way baseline load only, displacement only, load vs. time, displacement vs. time and/or load vs. displacement (with and without respect to time) curves are established for a specific configuration. Baseline information can be generated using lab tests, finite element analysis, or a combination of both. The baseline information is collected at discrete values of hole diameter, thickness, material, indenter style, or other important parameters. The baseline information need not be analyzed at all configurations, but only at logical points. In this way the response of specific cold-working applications can be interpolated or extrapolated using the discrete baseline data or through equations developed from the discrete data. Should the monitoring and feedback values for a specific cold-working application deviate significantly from the baseline curves then a signal of non-compliance is sent to the appropriate operator or expert system. The operator or expert system must then fix the non-compliance before proceeding. Deviations from the baseline may be used to generate troubleshooting information for the operator or expert system. For example, an indenter that has displaced significantly without a corresponding increase in load may have one of four or more potential problems; 1) indenter is missing from the driving head, 2) indenter is broken and is not engaging with structure, 3) machine is setup improperly or, 4) a sensor is malfunctioning. Using baseline curves of the material response to the StressWave cold-working allows various combinations of monitored parameters to be used for consistent processing.

[0035] Measurement of load and displacement can be made using sensors in the indenter, driving head, frame, or to equipment attached to the same, or non-attached equipment like optical, proximity or other remote sensors. Consideration must be made to where the measurement is made compared to the location of the desired input. Equipment compliance, repeatability, weight, etc. must be considered when generating or interpreting the measured value of load, pressure and/or displacement for any indenter component. Measurement can be made using load cells, strain gages, mechanical, or electronic displacement devices such as voltage displacement transducers.

[0036] An adjustable compound indenter that uses independent control may use pressurized hydraulic fluid to control the loads and displacements of the various indenter components. This type of device will be further discussed hereinbelow. The hydraulic fluid is supplied by either a single pump that acts on all the components or by separate pumps that act individually on the components. In any case the indenter and driving head software or database controls the hydraulic pressure to the indenter components. For a single hydraulic pump system the load to the individual indenter components is controlled both by the hydraulic pressure and the load bearing surface area of the specific indenter component. In some cases the pressure supplied to the individual components using a single pump arrangement may be modified using pressure relief valves. These pressure relief valves may be fixed at a specific level or electronically adjusted depending on the cold working requirements.

[0037] A multiple pump system where hydraulic fluid is supplied to the individual indenter components separately offers an alternate means of system control. This configuration provides fully independent movement and actuation of one or more of the indenter components. This offers advantages for those workpieces having different materials and/or differing layer thicknesses in the stackup.

[0038] For those driving heads and/or driving head software that lack a means for fully independent control it may be desirable to fit them with an indenter of adjustable indenter that has dependent control. Dependent control is a means by which the load and displacement of the indenter components are controlled by springs, media, mechanisms or other devices internal to the indenter. Dependent control can be achieved by using any device, medium, electricity, magnetic devices, etc., that transfers loads and/or displacements from one of the compound indenter components to any or all of the other compound indenter components or a mechanism or device that controls the other indenter components based on information from one component. Dependent control can also be achieved by changing any of the properties of the mechanisms, media and devices.

[0039] One example of dependent control for an adjustable compound indenter is where a spring connects the primary indenter to the secondary indenter. When the primary indenter is loaded or displaced directly by the indenter driver the secondary indenter is loaded by the spring connection. In this example the secondary indenter is loaded using a proportional percentage of the load from the primary indenter. The advantage to this type of control is the ability of the indenter to adjust the primary indenter length for varying loads and displacements without the need for a high degree of active control. Indenters of this type can be used on those machines and indenter drivers where the amount of active control capability on the driving head, driving head software and/or indenter is limited.

[0040] Spring types for this type of dependent control indenter include may include any combination of coil, flat, leaf, torsion, solid, and Belleville springs. The spring materials may include any combination of metals, piezo-electric materials, rubbers, plastics, elastomers, viscoelastic materials, fluids, or gases. The materials, devices, springs, spring-like materials and fluids may act in a linear or non-linear fashion; whichever is most suitable and advantages to a specific cold-working application. Passing them through an orifice or a series of restrictions may control behavior of some fluids. Springs and spring-like materials with desirable time dependent behavior may also be used for compound indenter control where a change in elastic modulus is associated with a specific rate of load. The term spring hereafter refers to any of the aforementioned spring materials and spring-like devices. Springs that stiffen with increase rates of load may be desirable for making the adjustable indenter more compact or able to provide higher loads to the workpiece. Springs that exhibit higher material properties in a closed and partially or completely filled cavity may also be advantageously used for control.

[0041] The primary indenter in one embodiment of the invention must have a flange or other load-bearing feature with sufficient surface area and strength to transfer load from the primary indenter through a spring or fluid to the secondary indenter sufficient for the secondary indenter to penetrate the workpiece surface. In the same way the secondary indenter load receiving surface must be of sufficient cross sectional size and strength to receive the load transferred to it. The spring or fluid must have sufficient mechanical properties and size for repeatably receiving load from the primary indenter and transferring it to the secondary indenter. A spring or fluid that does not have sufficient restoring force to return to its original shape, except after many load actuations is not generally suitable for use in a dependently controlled compound indenter. The change in cross-section and length of these types of materials allow the initial length of the primary indenter relative to the secondary indenter to “drift”. The drift in the length of the primary indenter typically reduces the load applied to the secondary indenter. Over time, a reduced load to the secondary indenter reduces the fatigue performance of the cold worked hole. Springs or fluids with limited restoring force may be used if their behavior can be predicted and accounted for.

[0042] For some applications the dimple depths on the front and back surface of the workpiece may require unequal depth or shape. The dependent control indenter can accommodate this requirement by simply changing the spring/fluid size or characteristics in one of the indenters. It can also be accomplished by changing the configuration of the primary and/or secondary indenters on one side. In this way a change in dimension or stiffness of one of the indenters allows for a different dimple shape. The foot of the indenter that imparts the shallower dimple can carry any excess load created by the side requiring a deeper dimple depth.

[0043] Specific Design Applications

[0044]FIGS. 1 through 3 show free body diagrams of three styles of compound indenters using dependent control and the load and resulting reaction forces on the primary and secondary indenters. Each compound indenter free body diagram reduces to a specific device, but the basic method of control is the same. The load or displacement as represented by the downwardly directed arrow controls the overall movement of the indenter. The reaction forces, R1 and R2, resulting from the indenter load or displacement are shown as upwardly directed arrows. R1 represents the reaction force for the primary indenter and R2 represents the reaction force for the secondary indenter. Two R2 reaction forces are illustrated, but in reality only one reaction force from the secondary indenter is present. The two R2 reaction force arrows better illustrate the actual indenter as the secondary indenter is typically coaxial to and surrounds the primary indenter. L_(initial) is the initial length of the primary indenter relative to the initial working surface of the secondary indenter. When the primary indenter is initially extended beyond the secondary indenter it is said to be “leading” the secondary indenter. A primary indenter with lead will contact the workpiece first. The primary indenter can also be retracted into a compound adjustable indenter. In this configuration the primary indenter lags the secondary indenter. Lead and lag are terms associated with the primary indenter relative to the secondary indenter. Another important parameter, L_(final) is the final length of the primary indenter at the maximum load and/or displacement. L_(final) and the length of the working portion (height) of the secondary indenter determine the overall dimple depth. The various indenter configurations and control methods described herein rely on controlling the overall dimple depth. This is necessary for achieving sufficient fatigue life improvement in the workpiece. Two other parameters are also shown; K1 is the spring constant for a spring attached to the primary indenter and K2 is the spring constant for a spring attached to the secondary indenter. K1 and K2 are named as spring constants, but may represent nonlinear and/or time dependent behavior as well.

[0045] A primary indenter with lead has the ability to remove itself from the dimple after cold working. The energy stored by the spring connected to the secondary indenter may have sufficient restoring force to pull the primary indenter from the dimple. Features on the primary indenter such as a relief angle, chamfer or radius on the working end, permanent or bonded coatings or applied lubricants will enhance the ability of the spring to remove the primary indenter from the dimple.

[0046] The load-displacement curves in FIGS. 1 through 3 have as x and y axes the displacement and load respectively. The reaction forces R1 and R2 and the load (sum of R1 and R2) as a function of displacement are shown. For illustration purposes the loads and displacements have been non-dimensionalized to have maximum values of 1.0. This is the value of the load required to displace both primary and secondary indenters to a depth necessary for achieving fatigue life improvement in the workpiece. The zero point, or origin, for both the load and displacement is defined as the instant the primary indenter touches the structure. L_(initial) can be seen as the difference between the origins of R1 and R2 as illustrated in FIG. 1. The initial slopes of the reaction forces are dependent on the spring constants of the springs attached to the indenter components or the elastic modulus of the indenter components. The initial slopes of indenter components attached to springs are typically less than those of indenter components attached solidly. It should be noted that these load displacement curves do not show the effect of machine compliance. This would have the effect of translating the origin of the reaction forces along either or both axes.

[0047]FIG. 1 shows the free body diagram and load-displacement curve of an integral one-piece compound indenter or solidly attached compound indenter components. The primary indenter typically “leads” the secondary indenter. L_(initial) is typically a fixed length.

[0048]FIG. 2 shows the free body diagram and load-displacement curve of an adjustable compound indenter where a spring is attached to the secondary indenter. As with the integral one-piece indenter the primary indenter typically leads the secondary indenter, but the lead length, L_(initial), is less. L_(initial) is typically smaller than the integral one-piece indenter so that there is sufficient force developed in the spring of the secondary indenter for working the surface by the time that the primary indenter has reached or nears its maximum load. L_(initial) may be negative for those applications requiring a high degree of force in the secondary indenter, i.e., the primary indenter lags the secondary indenter. For a lagging configuration the secondary indenter makes contact and starts to work the surface prior to the primary indenter.

[0049]FIG. 3 shows the free body diagram and load-displacement curve of an adjustable compound indenter where a spring is attached to the primary indenter. As with the integral one-piece indenter the primary indenter leads the secondary indenter, but the lead length is greater. L_(initial) is typically greater than the integral one-piece indenter so that there is enough force developed in the spring of the primary indenter sufficient for working the surface. As an alternative, the primary indenter function may be used for alignment purposes on large holes in thin structure. The primary indenter makes initial contact with the workpiece and aligns with a pre-existing drill center or other feature in the workpiece. As another alternative the primary indenter may create a drill center for subsequent alignment and machining of the hole. The secondary indenter then cold-works the workpiece while the primary indenter maintains alignment and reduces in length by way of the attaching spring.

[0050]FIGS. 4 through 6 show free body diagrams of three styles of adjustable compound indenters using independent control. Each compound indenter free body diagram reduces to a specific device, but the basic method of control is the same. The load and resulting reaction forces, R1 and R2, of the primary and secondary indenters are indicated as before. No load displacement graphs are shown as there are many more variations of load and displacement available to an independently controlled adjustable compound indenter. The load, or displacement, as represented by the downwardly directed arrow, and the device, controls the overall movement of the indenter. The load may also be considered as a reaction force as the load may be created by internal devices or through hydraulic or pneumatic pressure applied locally to the indenter. Device #1 is any equipment, mechanism, electromechanical device, pressurized cavity, and the like that actively controls and adjusts the primary indenter. A sensor may be attached to either the primary indenter or to Device #1 to monitor and measure load and/or displacement. Device #2 is any equipment, mechanism, electromechanical device, pressurized cavity, and the like that actively controls and adjusts the secondary indenter. A sensor may be attached to either the secondary indenter or to Device #2 to monitor and measure load and/or displacement. The reaction forces, R1 and R2, resulting from the device loads and/or displacements are shown as upwardly directed arrows. R1 represents the reaction force for the primary indenter and R2 represents the reaction force for the secondary indenter. Two R2 reaction forces are illustrated, but in reality only one reaction force from the secondary indenter is present. The two R2 reaction force arrows better illustrate the actual indenter as the secondary indenter is typically coaxial to and surrounds the primary indenter. L_(initial) is the initial length of the primary indenter relative to the initial working surface of the secondary indenter. Another important parameter (not shown), L_(final) is the final length of the primary indenter at the maximum load and/or displacement. L_(final) and the length of the working portion of the secondary indenter determine the overall dimple depth.

[0051]FIG. 8 shows a dependently controlled adjustable compound indenter where the cavity between the primary and secondary indenters is partially filled with a spring. The sidewalls of the indenter assembly are sufficiently stiff for containing any radial displacement of the spring. The flange area of the primary indenters and load receiving area of the secondary indenter are sized to carry the load transferred between the primary indenter and secondary indenter. Further, the spring is sized to carry the load required by the secondary indenter for penetrating the surface of the workpiece. In some applications only enough force to engage the primary indenter with the workpiece is used. This may be where the primary indenter is cold-working an area for a smaller diameter hole.

[0052]FIG. 9 shows a dependently controlled adjustable compound indenter where the cavity between the primary and secondary indenters is substantially filled with a fluid. The fluid transfers the load from the primary indenter to the secondary indenter. The sidewalls of the indenter assembly are sufficiently stiff for containing any radial displacement as a result of pressurizing the fluid. The flange area of the primary indenters and load receiving area of the secondary indenter are sized to carry the load transferred between the primary indenter and secondary indenter. Further, the surface areas of the components are sized to carry the pressure required by the secondary indenter for penetrating the surface of the workpiece. In some applications only enough force to engage the primary indenter with the workpiece is used. This may be where the primary indenter is cold-working an area for a smaller diameter hole.

[0053]FIG. 10 shows a dependently controlled adjustable compound indenter where the cavity between the primary and secondary indenters is substantially filled with a spring. This filling of the cavity allows for using spring materials who's mechanical properties advantageously improve under a semi or fully captivated condition. The sidewalls of the indenter assembly are sufficiently stiff for containing any radial displacement of the spring. The flange area of the primary indenter and load receiving area of the secondary indenter are sized to carry the load transferred between the primary indenter and secondary indenter. Further, the spring is sized to carry the load required by the secondary indenter for penetrating the surface of the workpiece. In some applications only enough force to engage the primary indenter with the workpiece is used. This may be where the primary indenter is cold-working an area for a smaller diameter hole.

[0054]FIG. 11 shows a dependently controlled adjustable compound indenter where the cavity between the primary and secondary indenters contains a spring. The length of the reduced diameter of the primary indenter of this embodiment is shorter that of the embodiments show in FIGS. 8 and 10 as the spring is positioned further away from the secondary indenter. This improves the overall resistance of the primary indenter to buckling. Additionally, it also provides room for a stop on the secondary indenter that prevents gross movement of the primary indenter should the spring fail. The area of the integral foot is also optionally sized to carry the maximum load of the driving head without any additional deformation to the work piece by the secondary indenter. The sidewalls of the indenter assembly are sufficiently stiff for containing any radial displacement of the spring. The flange area of the primary indenter and load receiving area of the secondary indenter are sized to carry the load transferred between the primary indenter and secondary indenter. Further, the spring is sized to carry the load required by the secondary indenter for penetrating the surface of the workpiece. In some applications only enough force to engage the primary indenter with the workpiece is used. This may be where the primary indenter is cold-working an area for a smaller diameter hole.

[0055]FIG. 12 shows a reduced profile, dependently controlled, adjustable compound indenter where the cavity between the primary and secondary indenters contains a spring. The length of the reduced diameter of the primary indenter of this embodiment is shorter that of the embodiments show in FIGS. 8 and 10 as the spring is positioned further away from the secondary indenter. This improves the overall resistance of the primary indenter to buckling. Additionally, it also provides room for a stop on the secondary indenter that prevents gross movement of the primary indenter should the spring fail. The area of the integral foot is also optionally sized to carry the maximum load of the driving head without any additional deformation to the work piece by the secondary indenter. The sidewalls of the indenter assembly are sufficiently stiff for containing any radial displacement of the spring. The flange area of the primary indenter and load receiving area of the secondary indenter are sized to carry the load transferred between the primary indenter and secondary indenter. Further, the spring is sized to carry the load required by the secondary indenter for penetrating the surface of the workpiece. In some applications only enough force to engage the primary indenter with the workpiece is used. This may be where the primary indenter is cold-working an area for a smaller diameter hole.

[0056]FIG. 13 shows the control limits for a typical compound indenter. Should the sensors record an exceedance of the limits for the load, displacement or both then a signal will notify an operator or expert system that a malfunction has occurred. The baseline curves to which the control limits can be determined by cold working representative structure using the indenters or through finite element analysis methods. The control limits are based on the allowable variation in dimple depth on the structure. Allowable dimple depth is determined through fatigue testing, x-ray diffraction analysis of the residual stress, finite element analysis or interpolation or extrapolation of known allowables.

[0057] Turning now to FIG. 14, and further to the information noted above, the main shaft of an indenter, whether a single indenter or a compound indenter, is usually supported by a press or via an electrically actuated impact tool driver. However, neither such apparatus is easily adjusted or suitable for finely limiting the total force which the indenter brings to bear on a selected metal workpiece. Generally, press driven mechanisms are designed for a selected length of travel, which results in relatively uniform displacement of the workpiece by the indenter. However, any variation in workpiece thickness, or in exact travel of the press, may dramatically change the actual residual compressive stress imparted in the workpiece. And, while electro-impact tools can be designed for a specific set of conditions, they are subject to some variation due to voltage fluctuations, or slight differences between tool construction.

[0058] Thus, it would be desirable in implementing widespread usage of the StressWave metal treatment process to provide an easily accomplished, precise, and reliable method for the control of the amount of force brought to bear on a workpiece. And, while it would be advantageous for the method to be applicable to when utilizing a single indenter, it would be even more advantageous to provide such in improvement for use in the application of compound indenters, that is, when an indenter has a point portion and at least a first shoulder portion which are independently displaceable.

[0059] A hydraulic control system for indenter systems has been developed and is disclosed herein. Load on an indenter is increased by increasing the pressure in a hydraulic reservoir acting on a piston. When the pressure reaches a desired limit, as calibrated by the pressure in the fluid reservoir responsive to the piston being driven by the indenter, then a pressure relief valve allows the fluid to flow from the cylinder to a hydraulic reservoir, thus allowing the indenter shaft to move away from the workpiece. Then, on recycle of the apparatus, an automatic control system reacts to such movement and pumps fluid back to the reservoir, and thus returns the indenter shaft to a pre-determined position. In one embodiment, the pressure relief valve is integrated into, or attached to, the indenter.

[0060] Indenter 10 comprises a frame 12 and therein a primary indenter shaft 14 supported through a suitable bearing by a piston 16 movable in a hydraulic cylinder 18. The primary indenter shaft 14 can be kept at a desired position by adjusting the amount of hydraulic fluid 20 in the cylinder 18. Pressurized hydraulic fluid is led from pump unit 22 through inlet line 24 into the hydraulic cylinder 18 to support the piston 16 and thereby the primary indenter shaft 14. The hydraulic fluid is normally led out from the hydraulic cylinder 18 through the line 26 and alternately through feedback adjustment 28 of pump unit 22, and then into tank 30. The pump unit 22 then takes fluid from the tank 30.

[0061] A pressure relief valve 32 is integrated to or preferably into the frame 12 of the indenter 10. When the desired load limit of the indenter 10 is reached, overpressure in the cylinder 18 opens the relief valve 32, and fluid flows from the inlet line 24 through a by-pass line 40.

[0062] The indenter 10 is also provided with a setting transducer 50, which detects the change of the primary indenter shaft 14 position. When a load limit situation is over, the primary indenter shaft 14 is automatically reset to the original position. This process is controlled by means of control unit 50 connected to the pump unit 22 and to the transducer 50.

[0063] When fabricated without any pressure accumulator, the system is simple, compact and easy to assemble and operate. Further, when the relief valve is joined without hoses to the indenter frame, the system response times are very short. This is an important advantage especially in cold climates, where the resistance in hoses is even more significant as temperature drops. Also, in an overload situation there is no counterpressure to the hydraulic fluid, which further increases the efficiency and reliability of the system.

[0064] The resetting transducer 50 may comprise of a toothed rack attached to the piston and of a corresponding gear wheel connected to an angle detector.

[0065] The pump unit 22, and the remainder of the hydraulic system are provided with normal auxiliary equipment necessary for reliable operation of such systems.

[0066]FIG. 14 also shows the use of a secondary indenter concentric to a primary indenter. The same components as just described may be utilized in conjunction therewith.

[0067] It is to be appreciated that the various aspects and embodiments of a method of the control of indenters during the manufacturing a fatigue life enhanced product as described herein are an important improvement in the state of the art. The method is simple to implement, and the products produced thereby will be robust, reliable, and susceptible to utilization of a variety of cold working techniques for various part configurations. Although only a few exemplary embodiments have been described in detail, various details are sufficiently set forth in the specification, in the drawing, and in the claims provided herein to enable one of ordinary skill in the art to make and use the invention(s), which need not be further described by additional writing in this detailed description.

[0068] Importantly, the aspects and embodiments described and claimed herein may be modified from those shown without materially departing from the novel teachings and advantages provided by this invention, and may be embodied in other specific forms without departing from the spirit or essential characteristics thereof. Therefore, the embodiments presented herein are to be considered in all respects as illustrative and not restrictive. As such, this disclosure is intended to cover the methods and structures described herein and not only structural equivalents thereof, but also equivalent structures. Numerous modifications and variations are possible in light of the above teachings. It is therefore to be understood that within the scope of the appended claims, the invention(s) may be practiced otherwise than as specifically described herein. Thus, the scope of the invention(s), as set forth in the appended claims, and as indicated by the drawing and by the foregoing description, is intended to include variations from the embodiments provided which are nevertheless described by the broad interpretation and range properly afforded to the plain meaning of the claims set forth below. 

1. Tooling for working a structure to improve the fatigue strength at a selected location in said structure, said structure comprising a first surface, a second surface, and a body therebetween, said tooling comprising: a first compound indenter, said first compound indenter comprising a first primary indenter, said first primary indenter comprising a contacting end for engagement with and deformation of a pre-selected portion of said first surface of said structure to impart a residual stress profile in said body of said structure; a first secondary indenter, said first secondary indenter comprising a contacting end for engagement with and deformation of a pre-selected portion of said first surface of said structure to impart a residual stress profile in said body of said structure, wherein said contacting end of said primary indenter comprises a first shaped surface having a preselected profile, and wherein said contacting end of said first secondary indenter comprises a second shaped surface having a preselected surface profile; and wherein said first primary indenter and said first secondary indenter are configured for companion engagement with said structure, and wherein first primary indenter acts on said first secondary indenter through a load transfer mechanism.
 2. Tooling as set forth in claim 1, wherein said first primary indenter further comprises a flange member for applying load to said load transfer mechanism.
 3. Tooling as set forth in claim 1 or in claim 2, wherein said load transfer mechanism comprises a spring.
 4. Tooling as set forth in claim 1 or in claim 2, wherein said load transfer mechanism comprises a substantially non-compressible fluid.
 5. Tooling as set forth in claim 2, wherein said first primary indenter comprises a sufficiently large flange surface area so that the load transferred is sufficient for the secondary indenter to penetrate a preselected workpiece.
 6. Tooling as set forth in claim 2, wherein said first secondary indenter comprises sufficiently load receiving surface area to receive load from said load mechanism so that the load transferred is sufficient for the secondary indenter to penetrate a preselected workpiece.
 7. Tooling as set forth in claim 1, wherein said load transfer mechanism additionally performs the function of removing said first primary indenter from a dimple in said workpiece.
 8. Tooling as set forth in claim 7, wherein said load transfer mechanism additionally performs the function of removing said second primary indenter from a dimple in said workpiece.
 9. Tooling as set forth in claim 7 or in claim 8, wherein said load transfer mechanism comprises a spring.
 10. A hydraulic control system for an indenter having a frame, in which a primary indenter shaft is supported hydraulically by a first piston movable in a first hydraulic cylinder, said system comprising: a tank for hydraulic fluid, a first pump unit and first line which leads hydraulic fluid from the tank into the first cylinder; a first return line which leads hydraulic fluid from the first cylinder into the tank, and a first pressure relief valve connected to the first inlet line and having associated therewith an opening pressure at which the first relief valve opens, and a first by-pass line connected between the first relief valve and tank, such that when the pressure in the first hydraulic cylinder exceeds the opening pressure of the first relief valve, fluid flows from the first hydraulic cylinder through the first relief and through a first return line to the tank.
 11. A system in accordance with claim 10, wherein the first by-pass line is integrated the indenter frame.
 12. A system in accordance with claim 11, further comprising a first transducer which monitors a height position of the primary indenter shaft, and a control unit connected to the first transducer and to the first pump unit such that when the pressure in the first hydraulic cylinder is below the opening pressure of the first relief valve, the control unit operates the first pump unit to automatically position the primary indenter shaft at a predetermined position.
 13. A system in accordance with claim 11, wherein the first by-pass line is placed in the frame of the indenter.
 14. A system in accordance with claim 13, further comprising a first transducer which monitors a height position of the primary indenter shaft in the indenter frame, and a control unit connected to the first transducer and to the first pump unit such that when the pressure in the first hydraulic cylinder is below the opening pressure of the first relief valve, the control unit operates the first pump unit to automatically position the primary indenter shaft at a predetermined position.
 15. A system in accordance with claim 10, further comprising a first transducer which monitors a height position of the primary indenter shaft in the indenter frame, and a control unit connected to the first transducer and to the first pump unit such that when the pressure in the first hydraulic cylinder is below the opening pressure of the first relief valve, the control unit operates the first pump unit to automatically position the primary indenter shaft at a predetermined position.
 16. A system in accordance with claim 10, wherein the first pressure relief valve is integrated into the indenter frame.
 17. A system according to claim 10, further comprising a secondary indenter having a secondary indenter shaft portion, said secondary indenter shaft hydraulically supported by a second piston moveable in a second hydraulic cylinder; a second pump unit and second line which leads hydraulic fluid from the tank into the second hydraulic cylinder; a second return line which leads hydraulic fluid from the second cylinder into the tank, and a second pressure relief valve connected to the second inlet line and having associated therewith an opening pressure at which the second relief valve opens, and a second by-pass line connected between the second relief valve and tank, such that when the pressure in the second hydraulic cylinder exceeds the opening pressure of the second relief valve, fluid flows from the second hydraulic cylinder through the second relief valve and through a second return line to the tank.
 18. A system in accordance with claim 17, wherein the second by-pass line is integrated the indenter frame.
 19. A system in accordance with claim 18, further comprising a second transducer which monitors a height position of the secondary indenter shaft, and a control unit connected to the second transducer and to the second pump unit such that when the pressure in the second hydraulic cylinder is below the opening pressure of the second relief valve, the control unit operates the second pump unit to automatically position the secondary indenter shaft at a predetermined position.
 20. A system in accordance with claim 19, wherein the second by-pass line is placed in the frame of the indenter.
 21. A system in accordance with claim 20, further comprising a second transducer which monitors a height position of the secondary indenter shaft in the indenter frame, and a control unit connected to the second transducer and to the second pump unit such that when the pressure in the second hydraulic cylinder is below the opening pressure of the second relief valve, the control unit operates the second pump unit to automatically position the secondary indenter shaft at a predetermined position.
 22. A system in accordance with claim 17, further comprising a second transducer which monitors a height position of the secondary indenter shaft in the indenter frame, and a control unit connected to the second transducer and to the second pump unit such that when the pressure in the second hydraulic cylinder is below the opening pressure of the second relief valve, the control unit operates the second pump unit to automatically position the secondary indenter shaft at a predetermined position.
 23. A system in accordance with claim 17, wherein the second pressure relief valve is integrated into the indenter frame. 